The compound that is used in smoke screens is

Which of the following is $NOT$ a polymorphic form of silica $(SiO_2)$?

For bleaching powder,which of the following statements is incorrect?

$P_4O_{10}$ has short and long $P-O$ bonds. The number of short $P-O$ bonds in this compound is

Identify the incorrect statement about $SiO_2$ (silica) from the following.

The correct statements with respect to halides of group $14$ elements are:
$I$. Both $CCl_4$ and $SiCl_4$ undergo hydrolysis
$II$. $GeX_4$ is more stable than $GeX_2$
$III$. $PbX_4$ is less stable than $PbX_2$
$IV$. Stability of dihalides decreases down the group.